In an RF broadcast transmitting system, such as a VHF FM transmitter, employing tubes, considerable time may be spent during the set-up operation for neutralizing the cavity. The cavity is an enclosed cavity that contains the transmitting tube, such as a tetrode. Such a tube employs at least a cathode, an anode and a grid. The tube exhibits an internal grid to anode capacitance which provides an error factor during the operation. Consequently, it is important during the set-up procedure to neutralize this internal capacitance and this is frequently referred to as neutralizing the cavity.
It is common to employ feedback capacitors extending from the anode to the primary of a transformer, the secondary of which applies an opposing voltage to the grid for the purposes of neutralizing the cavity. This is a time consuming procedure because entry must be gained into the cavity in order to adjust the feedback capacitors therein and then closing the cavity and making a measurement. This process is repeated several times until satisfactory results towards gaining neutralization are obtained.
Other approaches to gain cavity neutralization have included adjusting screen inductance. This also requires entry into the cavity.
It is desirable, therefore, to obtain cavity neutralization in a manner that is external to the cavity and thereby reduce off air time and manufacturing expenses. Such a method of neutralization would be more stable over time than such prior art methods of adjusting the feedback capacitance.
In addition to the features noted above, the present invention minimizes dirt collection and arcing problems that have been associated with neutralization methods, thus far employing variable capacitors. Additionally, the invention places the anode sampling capacitors at DC ground potential and this will protect the control grid in the event of plate arcing. Also, the invention minimizes the need to add a high current, capacitor divider at the ground end of the grid tuning inductors which may drift in value during warm-up operation. The inventive approach allows the neutralization technique to be adjusted from outside the cavity without disturbing the cavity resonance. Tube changes can be made quickly and manufacturing test time can be greatly reduced employing the invention.